1. Field of the Invention
The present invention relates to technology for embedding/reading additional information such as copyright information and editing information into/from various data such as image data including moving images and still images, music data, and audio data. More particularly, the present invention relates to a digital watermark embedding (may also be referred to as “data hiding”) apparatus and method and a computer program for embedding a digital watermark as additional information imperceptible to normal visual or auditory observation into image data or audio data.
2. Description of the Related Art
As digital technology progresses, digital recorder/players that do not cause deterioration in image quality and sound quality due to repetitive recording and playing have become widely used. At the same time, various types of digital content such as images and music content have become distributable through media such as digital VCRs (videocassette recorders), DVDs (digital versatile disks), and CD (compact discs) and networks.
Unlike analog recording and playing, digital recording and playing do not cause deterioration of data even when recording and playing are repeatedly performed. As a result, the same quality as that of the original data can be maintained. The widely spread use of digital recording and playing technology allows for an excess of illegal copying. This is a serious problem in view of copyright protection.
In order to protect digital content against copyright infringement by illegal copying, the following scheme is devised. Specifically, the scheme involves adding copy control information to digital content, reading the copy control information when the content is recorded or played, and performing processing in accordance with the read copy control information, thus preventing illegal copying.
There are various content control schemes. A typical scheme employs, for example, a copy generation management system (CGMS). For an analog video signal (referred to as CGMS-A), the CGMS scheme superimposes copy control information, that is, two bits of 20-bit additional information to be superimposed on an effective video portion of a specific horizontal period, e.g., the 20th horizontal period in case of an NTSC signal, in a vertical blanking interval of a luminance signal. For a digital video signal (referred to as CGMS-D), the CGMS scheme transmits the digital video signal (digital video data) and two bits of copy control information serving as additional information to be inserted into the digital video data.
The two-bit information used in the CGMS scheme (referred to as CGMS information) has the following meanings: “00” means that copying is permitted, “10” means that one copying is permitted (one generation is permitted), and “11” means that copying is prohibited (copying is absolutely prohibited).
The CGMS scheme described above is an example of a typical copy control scheme. There are other schemes for protecting content copyright. For example, digital broadcasting performed by a broadcast station stores a digital copy control descriptor in program schedule information, that is, service information (SI), included in a transport stream (TS) packet forming digital data. When recording data received by a receiver in a recorder, copy generation control in accordance with the descriptor is performed.
The foregoing control information is added as bit data to the header of content and cannot completely eliminate possible illegal alteration of the added data. Digital watermarking is advantageous in eliminating the possibility of data alteration. When content (image data or audio data) is normally played, a digital watermark is imperceptible to vision and auditory senses. Digital watermarks can only be detected/embedded by execution of a specific algorithm or processing by a specific device. A digital watermark is detected when content is processed by a receiver or recorder/player, and processing in accordance with the digital watermark is performed, thus making more reliable control possible.
Information that can be embedded in content by digital watermarking includes not only the above-described copy control information, but also various information such as content copyright information, content processing information, content format information, content editing information, and content playing information.
Information such as a digital watermark that is directly superimposed on an information signal has a strong alteration resistance and is expected to serve as secure additional information. In digital watermarking technology, the quality of a watermarked image may become a problem. Since digital watermarking performs direct signal processing of data to be watermarked (e.g., an image), the quality of the image may deteriorate or the statistic characteristics of the image may be biased, leading to deterioration of the original image. Digital watermarking that does not cause adverse effects on content is highly demanded.
An example of a known method for preventing deterioration of original data involves setting digital watermark embedding parameters in individual data areas in the original data such as an image to be watermarked, and subsequently, again setting a digital watermark superimposing level adjusting parameter (global parameter) in the entire data area taking into consideration the deterioration in image quality.
There are various methods for controlling embedding of digital watermarks. In one method, an amount embedded is increased by utilizing the characteristics of an image. In another method, a digital watermark is embedded at the minimum embedding level so that priority may be given to image quality. There is not yet a digital watermark embedding algorithm, commonly applicable to all information signals such as image or audio data to be watermarked, for both achieving a sufficient digital watermark detecting accuracy and preventing deterioration in image quality. By embedding a digital watermark, the quality of a data signal such as image or audio data may deteriorate, or the reliability of detecting an embedded digital watermark may be reduced.
In the foregoing digital watermark superimposing level controlling method using the global parameter, a first algorithm for determining initial embedding parameters determines substantially an optimal level of superimposing a digital watermark in each data area. Subsequently, the final adjustment involving the global parameter is performed. The optimal parameters initialized by the first algorithm in individual data areas are forcedly changed by setting the global parameter that takes into consideration deterioration in image quality.
Setting the global parameter may not be favorable in view of digital watermark detection. In other words, the initial optimal parameters are set so as to maintain the superimposing level that ensures digital watermark detection. When the initial optimal parameters are changed by setting the global parameter, the sufficient detection level may not be achieved.